1. Field of the Invention
The present invention relates to a rare earth iron garnet single crystal film which has very high magnetic hysteresis, and more particularly to a bismuth-substituted rare earth iron garnet single crystal film having saturation magnetic field higher than its nucleation field over a wide range of its practical operating temperatures. Such a single crystal film functions properly as a Faraday rotator without an external magnetic field applied thereto and is therefore suitable for an optical isolator.
2. Description of Related Art
In recent years, the research of optical isolators, optical switches, and magneto-optic sensors which use a bismuth-substituted rare earth iron garnet single crystal film has been intensively carried out. Faraday effect is a magneto-optic effect in which the plane of polarization of light is rotated when the light passes through a Faraday element that exhibits the Faraday effect, i.e., a Faraday rotator made of a material such as bismuth-stubstituted rare earth iron garnet single crystal films.
In general, the Faraday rotation angle increases with the intensity of an external magnetic field applied to the Faraday rotator.
FIG. 1 is a model representation of a magnetic property of a bismuth-substituted rare earth iron garnet single crystal film having no hysteresis. As shown in FIG. 1, a bismuth substituted rare earth iron garnet single crystal film exhibits a saturated rotation angle when the external magnetic field exceeds a certain level. The magnetic field strength at which the Faraday rotation reaches a constant value is referred to as saturation magnetic field (Hs). Then, the Faraday rotation decreases with decreasing intensity of the external magnetic field, and becomes zero when the magnetic field becomes zero. In other words, the Faraday rotation traces a path o-a-b-c-b-a-o.
Actually, the Faraday rotation shows some magnetic hysteresis so that the Faraday rotation traces a path o-a-b-c-b-b'-a-o as shown in FIG. 1. There has been found a phenomenon that some bismuth-substituted rare earth iron garnet single crystal films having a chemical formula (YBi).sub.3 (FeAl).sub.5 O.sub.12 ! retain saturated Faraday rotation angles even if an external magnetic field is applied which is of substantially the same strength but of an opposite direction to the magnetic field by which the single crystal films were magnetically saturated, i.e., the magnetization curve follows the path o-a-b-c-b-d-e-f-e (Journal of Applied Physics, Vol. 55(1984), pp1052-1061).
FIG. 2 is a model representation of a magnetic property of a bismuth-substituted rare earth iron garnet single crystal film having a large square magnetic hysteresis loop that exceeds the saturation magnetic fried (Hs).
Referring to FIG. 2, the magnetic field strength at which the Faraday rotation angle is inverted in polarity is referred to as nucleation field (Hn). The difference between the Hs and Hn is the size of magnetic hysteresis.
A hysteresis curve of magnetization having large magnetic hysteresis such as that shown in FIG. 2 is referred to as square hysteresis loop.
Magneto-optic materials having a square magnetic hysteresis loop serve as a Faraday rotator that shows the Faraday effect even when no external magnetic field is applied thereto. Thus, a bismuth-substituted rare earth iron garnet single crystal film having a square hysteresis loop, when used as a Faraday rotator, does not need a permanent magnet which is usually required when manufacturing an optical isolator. Thus, such a film lends itself to miniaturizing the device and minimizing the cost of the optical isolator.
The inventors of the present invention found that the (GdYBi).sub.3 (FeGa).sub.5 O.sub.12 developed as a bismuth-substituted rare earth iron garnet single crystal film for optical switches exhibits a square hysteresis loop, and investigated the crystal film to determine whether the material could be used as a magnet-free Faraday rotator for optical isolators.
The inventors investigated the characteristics of a (GdYBi).sub.3 (FeGa).sub.5 O.sub.12 single crystal film, and found a problem that the material exhibits a square hysteresis loop at temperatures near room temperature but even a weak external magnetic field applied to the material is enough to cause the square hysteresis to disappear or to cause the Faraday rotation angle to get out of saturation at temperatures higher than +50.degree. C.
The inventors conducted research on the bismuth-substituted rare earth iron garnet single crystal film proposed in Japanese Patent Preliminary Publication (KOKAI) No. 6-222311, and found that the material magnetically saturated in one direction showed a Faraday rotation angle of an inverted polarity when placed in an external magnetic field of about 50 Oe in the opposite direction, and the material could maintain the square hysteresis only over a narrow temperature range.
Optical isolators are usually used in an environment of temperatures in the range from -20 to +50.degree. C. In addition, some external magnetic field may exist near the location where an optical isolator is installed. Thus, it is required that the material retains its saturated Faraday rotation angle when the material is placed in external magnetic fields up to 50 Oe and preferably 100 Oe.